Optical characteristic measuring apparatus, method and recording medium

ABSTRACT

The object of the present invention is to provide apparatuses that enable to enlarge the range of modulation frequencies that modulate the variable wavelength light generated by the light source without prejudice to the measurement of optical characteristics.  
     In order to achieve said object, a modified modulation frequency computing section  44  for computing modified modulation frequencies fi by multiplying by the initial modulation frequency fmin the value obtained by dividing the given phase value π by the phase difference between the first phase φmin_i of the transmitted light resulting from the transmission through the DUT  30  of the incident light of the first wavelength λi modulated by the initial modulation frequency fmin and the second phase φmin_i+1 of the transmitted light resulting from the transmission through the DUT  30  of the incident light of the second wavelength λi+1 modulated by the initial modulation frequency fmin, and a modified modulation frequency setting section  46  for setting the modified modulation frequency fi as the frequency of the modulating signal are included so that the frequencies for modulating the incident light may be wider in range than the initial modulation frequency fmin and that the phase difference between φi and φi+1 may be kept at a value equal to or below the given phase value π, and thus the precision of measuring phase differences can be enhanced.

TECHNICAL FIELD

[0001] The present invention relates to the measurement of thedispersion characteristic of optical fibers and other optical devices,and in particular to the determination of the frequency of modulatingthe incident light to optical devices.

BACKGROUND ART

[0002]FIG. 12 is a block diagram showing the configuration of an opticalcharacteristic measuring apparatus according to the prior art. As shownin FIG. 12, the measuring system is divided into a light source system10 and a characteristic measurement system 20. A variable wavelengthlight source 12 of the light source system 10 varies the wavelength togenerate a light (variable wavelength light) having wavelengths of λiand λi+1. The variable wavelength light will be modulated by a lightmodulator 14. The light modulator 14 includes LN (lithium niobate). Thelight modulator 14 receives an electrical signal having a frequency offi from a modulation power supply 16 and modulates the variablewavelength light with the frequency fi.

[0003] The light outputted from the light modulator 14 is introducedinto an optical fiber or other DUT (device under test) 30. Thetransmitted light having transmitted the DUT 30 will be supplied to anoptical/electrical converter 22 of the characteristic measuring system20. The optical/electrical converter 22 proceeds to anoptical/electrical conversion of the transmitted light and outputs to aphase comparator 24. The phase comparator 24 measures the phase of theoutput signal of the optical/electrical converter 22 with reference tothe electrical signal produced by the modulation power supply 16. Here,the phase when the incident light wavelength is λi will be representedby φi and the phase when the incident light wavelength is λi+1 will berepresented by φi+1. The characteristic computing section 26 willcompute the wavelength dispersion characteristic and othercharacteristics of the DUT 30 from φi and φi+1.

[0004] The operation of the characteristic computing section 26 will bedescribed with reference to the phase-wavelength diagram shown in FIG.13. When φi+1−φi is represented by Δφ, the group delay time is computedfrom Δφ and the modulation frequency fi, and then the wavelengthdispersion is computed therefrom.

[0005] Here, the range of phase difference that can be measured from thephase comparator 24 extends from −π to π. Therefore, it is preferablethat φi+1−φi would be within the range extending from −π to π. This isbecause any large modulation frequency fi can easily exceed the range of−π to π. In other words, when the same time difference is expressed bythe phase difference, the bigger the frequency is, the cycle is shorter,and when it is expressed by the phase difference, the cycle will belonger. For example, when the time difference is {fraction (1/50)}secs., if the frequency is 1 Hz, the range of phase difference is only0.04π, but if the frequency is 50 Hz, it will be 2π. Therefore, themodulation frequency fi should be lowered to the minimum possible, andthe wavelength λ of the incident light should be varied.

[0006] However, in order to measure Δφ with a high precision, it ispreferable that the modulation frequency fi has a high value. This isdue to the fact that, when a same time difference is expressed with aphase difference, the larger the frequency is, the shorter the cyclebecomes, and when it is expressed with a phased difference, the cyclewill be greater.

[0007] Therefore, the present invention has an object of providingdevices enabling to enlarge the modulation frequency that modulates avariable length wavelength generated by the light source without makingproblem with respect to the measurement of the optical characteristic.

DISCLOSURE OF INVENTION

[0008] According to the present invention as described in claim 1, anapparatus for measuring the characteristics of device under test thattransmits light includes: a variable wavelength light source forgenerating a variable wavelength light; a wavelength setting unit forsetting the variable wavelength light at a first wavelength and a secondwavelength; an initial modulation frequency setting unit for setting theinitial modulation frequency for modulation; a modulating signalgenerating unit for generating a modulating signal of a set modulationfrequency; an optical modulating unit for receiving the input of themodulating signal and modulating the variable wavelength light with thefrequency of the modulating signal; a phase measuring unit for measuringa first phase of a transmitted light, which is obtained by thetransmission through the device under test of an incident light havingthe first wavelength and a second phase of the transmitted light, whichis obtained by the transmission through the device under test of anincident light having the second wavelength; a modified modulationfrequency computing unit for computing a modified modulation frequencyby multiplying the value, which is obtained by dividing the given phasevalue by the phase difference between the first phase and the secondphase, by the initial modulation frequency; and a modified modulationfrequency setting unit for setting the modified modulation frequency asthe frequency of the modulating signal, wherein the characteristics ofdevice under test are measured on the basis of the transmitted lightresulting from the transmission through the device under test of theincident light modulated by a frequency set by the modified modulationfrequency setting unit.

[0009] The initial modulation frequency is limited to a small value toinsure that the phase difference between the first phase and the secondphase will be less than the given phase value, for example, π. However,the modified modulation frequency computing unit enables to compute amodified modulation frequency that causes the phase difference betweenthe transmitted light corresponding to the first wavelength and thetransmitted light corresponding to the second wavelength to coincidewith the given phase value. Therefore, if the frequency modulating theincident light is chosen as the modified modulation frequency by themodified modulation frequency setting unit, the phase difference betweenthe transmitted light corresponding to the first wavelength and thetransmitted light corresponding to the second wavelength will be thegiven phase value. Therefore, it is possible to measure the phasedifference. And further as the frequency for modulating the incidentlight can be increased, the measurement precision of the phasedifference can be improved.

[0010] The present invention as described in claim 2, is the opticalcharacteristic measuring apparatus according to claim 1, wherein theinitial modulation frequency setting unit sets the minimum initialmodulation frequency and the initial modulation frequencies other thanthe minimum initial modulation frequency; the modified modulationfrequency computing unit computes a modified modulation frequency bymultiplying by the minimum initial modulation frequency the valueobtained by dividing the given phase value by the phase differencebetween the first phase and the second phase of the transmitted lightresulting from the transmission through the device under test of theincident light modulated by the minimum initial modulation frequency;and the modified modulation frequency setting unit sets the maximum theinitial modulation frequency among the initial modulation frequenciesequal to or below the modified modulation frequencies as the frequencyof the modulating signal.

[0011] The initial modulation frequency is limited to a small value toinsure that the phase difference between the first phase and the secondphase will be less than the given phase value, for example, π. However,the modified modulation frequency computing unit enables to compute amodified modulation frequency that causes the phase difference betweenthe transmitted light corresponding to the first wavelength and thetransmitted light corresponding to the second wavelength to coincidewith the given phase value. Therefore, if the maximum initial modulationfrequency among the initial modulation frequencies below the modifiedmodulation frequency is set as the modified modulation frequency by themodified modulation frequency setting unit, the phase difference betweenthe transmitted light corresponding to the first wavelength and thetransmitted light corresponding to the second wavelength will be thegiven phase value. Therefore, it is possible to measure the phasedifference. And further as the frequency for modulating the incidentlight can be increased, the measurement precision of the phasedifference can be improved.

[0012] The present invention as described in claim 3, is the opticalcharacteristic measuring apparatus according to claim 1 or 2, whereinthere are a plurality of first wavelengths and a plurality of secondwavelengths.

[0013] The present invention as described in claim 4, is the opticalcharacteristic measuring apparatus according to claim 3 wherein, theintervals between the first wavelength and the second wavelength areequal, and the second wavelength is taken as the first wavelength andfurthermore another second wavelength is taken so that the intervalsbetween the first wavelength and the second wavelength are equal.

[0014] The present invention as described in claim 5, is the opticalcharacteristics measuring apparatus according to claim 3 or 4, whereinafter completing the setting of the first wavelength and the secondwavelength, the modified modulation frequency setting unit sets themodified modulation frequency as the frequency of the modulating signal.

[0015] The present invention as described in claim 6, is the opticalcharacteristics measuring apparatus according to claim 3 or 4, whereinevery time when the first wavelength and the second wavelength are set,the modified modulation frequency setting unit sets the modifiedmodulation frequency as the frequency of the modulating signal.

[0016] According to the present invention as described in claim 7, theoptical characteristics measuring apparatus according to either one ofclaims 1 to 6, includes an optical/electrical conversion unit foroutputting electrical signals obtained by optical/electrical conversionof the transmitted light to the phase measuring unit.

[0017] The present invention as described in claim 8, is the opticalcharacteristics measuring apparatus according to either one of claims 1to 6, wherein the phase measuring unit measures the phase differencebetween the modulating signal and the transmitted light.

[0018] According to the present invention as described in claim 9, theoptical characteristics measuring apparatus according to either one ofclaims 1 to 6, includes a characteristic computing unit for computingthe group delay or the wavelength dispersion of the device under test byunit of the phase difference measured by the phase measuring unit.

[0019] According to the present invention as described in claim 10, amethod for measuring the characteristics of device under test thattransmits light includes: a variable wavelength light generating stepfor generating a variable wavelength light; a wavelength setting stepfor setting the variable wavelength light at a first wavelength and asecond wavelength; an initial modulation frequency setting step forsetting the initial modulation frequency for modulation; a modulatingsignal generating step for generating a modulating signal of a setmodulation frequency; an optical modulating step for receiving the inputof the modulating signal and modulating the variable wavelength lightwith the frequency of the modulating signal; a phase measuring step formeasuring the first phase of the transmitted light, which is obtained bythe transmission through the device under test of an incident lighthaving the first wavelength and the second phase of the transmittedlight, which is obtained by the transmission through the device undertest of an incident light having the second wavelength; a modifiedmodulation frequency computing step for computing a modified modulationfrequency by multiplying the value, which is obtained by dividing thegiven phase value by the phase difference between the first phase andthe second phase, by the initial modulation frequency; and a modifiedmodulation frequency setting step for setting the modified modulationfrequency as the frequency of the modulating signal, wherein thecharacteristics of device under test are measured on the basis of thetransmitted light resulting from the transmission through the deviceunder test of the incident light modulated by a frequency set by themodified modulation frequency setting step.

[0020] The present invention as described in claim 11, is the opticalcharacteristic measuring method according to claim 10, wherein theinitial modulation frequency setting step sets the minimum initialmodulation frequency and the initial modulation frequencies other thanthe minimum initial modulation frequency; the modified modulationfrequency computing step computes a modified modulation frequency bymultiplying by the minimum initial modulation frequency the valueobtained by dividing the given phase value by the phase differencebetween the first phase and the second phase of the transmitted lightresulting from the transmission through the device under test of theincident light modulated by the minimum initial modulation frequency;and the modified modulation frequency setting step sets the maximum theinitial modulation frequency among the initial modulation frequenciesequal to or below the modified modulation frequencies as the frequencyof the modulating signal.

[0021] The present invention as described in claim 12, is acomputer-readable medium having a program of instructions for executionby the computer to perform a characteristics measuring process formeasuring characteristics of device under test that transmits light, thecharacteristics measuring process including: a variable wavelength lightgenerating process for generating a variable wavelength light; awavelength setting process for setting the variable wavelength light ata first wavelength and a second wavelength; an initial modulationfrequency setting process for setting the initial modulation frequencyfor modulation; a modulating signal generating process for generating amodulating signal of a set modulation frequency; an optical modulatingprocess for receiving the input of the modulating signal and modulatingthe variable wavelength light with the frequency of the modulatingsignal; a phase measuring process for measuring the first phase of thetransmitted light, which is obtained by the transmission through thedevice under test of an incident light having the first wavelength andthe second phase of the transmitted light, which is obtained by thetransmission through the device under test of an incident light havingthe second wavelength; a modified modulation frequency computing processfor computing a modified modulation frequency by multiplying the value,which is obtained by dividing the given phase value by the phasedifference between the first phase and the second phase, by the initialmodulation frequency; and a modified modulation frequency settingprocess for setting the modified modulation frequency as the frequencyof the modulating signal, wherein the characteristics of device undertest are measured on the basis of the transmitted light resulting fromthe transmission through the device under test of the incident lightmodulated by a frequency set by the modified modulation frequencysetting process.

[0022] The present invention as described in claim 13, is thecomputer-readable medium according to claim 12, wherein the initialmodulation frequency setting process sets the minimum initial modulationfrequency and the initial modulation frequencies other than the minimuminitial modulation frequency; the modified modulation frequencycomputing process computes a modified modulation frequency bymultiplying by the minimum initial modulation frequency the valueobtained by dividing the given phase value by the phase differencebetween the first phase and the second phase of the transmitted lightresulting from the transmission through the device under test of theincident light modulated by the minimum initial modulation frequency;and the modified modulation frequency setting process sets the maximumthe initial modulation frequency among the initial modulationfrequencies equal to or below the modified modulation frequencies as thefrequency of the modulating signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a block diagram showing the configuration of an opticalcharacteristic measuring apparatus related to the first preferredembodiment of the present invention.

[0024]FIG. 2 is an illustration describing the principle of how themodified modulation frequency computing section 44 computes the modifiedmodulation frequency fi.

[0025]FIG. 3 is a flowchart showing the operation of the first preferredembodiment of the present invention.

[0026]FIG. 4 is a block diagram showing the configuration of an opticalcharacteristic measuring apparatus related to the second preferredembodiment of the present invention.

[0027]FIG. 5 is a flowchart showing the operation of the secondpreferred embodiment.

[0028]FIG. 6 is a phase-wavelength diagram showing the operation of thesecond preferred embodiment.

[0029]FIG. 7 shows the relationship between the phases measured by thephase comparator 24 and the wavelengths of the variable wavelength lightwhen there are three or more wavelengths of the variable wavelengthlight in the third preferred embodiment through the sixth preferredembodiment.

[0030]FIG. 8 is a flowchart showing the operation of the third preferredembodiment.

[0031]FIG. 9 is a flowchart showing the operation of the fourthpreferred embodiment.

[0032]FIG. 10 is a flowchart showing the operation of the fifthpreferred embodiment.

[0033]FIG. 11 is a flowchart showing the operation of the sixthpreferred embodiment.

[0034]FIG. 12 is a block diagram showing the configuration of an opticalcharacteristic measuring apparatus according to the prior art.

[0035]FIG. 13 is a phase-wavelength diagram according to the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

[0036] The preferred embodiments of the present invention are describedwith reference to drawings.

[0037] The First Preferred Embodiment

[0038]FIG. 1 is a block diagram showing the configuration of the opticalcharacteristic measuring apparatus related to the first preferredembodiment of the present invention. The optical characteristicmeasuring apparatus related to the preferred embodiments of the presentinvention includes a light source system 10 for introducing light to theDUT 30, a characteristic measuring system 20 for receiving light havingtransmitted the DUT 30 and measuring the characteristics of the DUT 30,and a modulation frequency setting system 40 for setting the modulationfrequency.

[0039] The light source system 10 includes a variable wavelength lightsource 12, an optical modulator 14, a modulating power source 16, and awavelength setting section 18.

[0040] The variable wavelength light source 12 generates a variablewavelength light. The wavelength of the variable wavelength light variesdiscretely by means of the wavelength setting section 18 between thefirst wavelength of λi and the second wavelength of λi+1. The lightmodulator 14 modulates the variable wavelength light with the frequencyof electrical signals generated by the modulation power supply 16 andsupplies the variable wavelength light to the DUT 30. In the meanwhile,the light modulator 14 includes LN (lithium niobate). The modulationpower source 16 generates electrical signals for modulating thefrequencies set by the modulation frequency setting system 40. Theelectrical signals for modulation are supplied to the light modulator 14and the phase comparator 24 described below. The wavelength settingsection 18 sets discretely the wavelength of variable wavelength lightsat the first wavelength λi and the second wavelength λi+1.

[0041] The DUT 30 is for example an optical fiber. The incident lightsupplied to the DUT 30 transmits the DUT 30. The incident lighttransmitting the DUT 30 is called “transmitted light.”

[0042] The characteristic measuring system 20 includes anoptical/electrical converter 22, a phase comparator 24, a characteristiccomputing section 26 and a modified phase recording section 28.

[0043] The optical/electrical converter 22 converts the transmittedlight through the optical/electrical conversion process and generateselectrical signals which are outputted to the phase comparator 24. Thephase comparator 24 measures the phase difference between the electricalsignals obtained by converting the transmitted light through theoptical/electrical conversion process and the electrical signals formodulation. The modified phase recording section 28 records the firstmodified phase φi and the second modified phase φi+1 respectivelycorresponding to the first wavelength λi and the second wavelength λi+1of the incident light when the modified modulation frequency settingsection 46 described later sets the frequency of the modulation powersource 16 at fi. The characteristic computing section 26 computes thegroup delay characteristic and the wavelength dispersion characteristicof the DUT 30 from the first modified phase φi and the second modifiedphase φi+1 recorded at the modified phase recording section 28. Thegroup delay characteristic can be computed from the relationship betweenthe phase measured by the phase comparator 24 and the modulationfrequency fi. The wavelength dispersion characteristic can be obtainedby differentiating the group delay characteristic by the wavelength.

[0044] The modulation frequency setting system 40 includes an initialphase recording section 42, a modified modulation frequency computingsection 44, a modified modulation frequency setting section 46, and aninitial modulation frequency setting section 48. The initial phaserecording section 42 records the first initial phase φmin_i and thesecond initial phase φmin_i+1 respectively corresponding to the firstwavelength λi and the second wavelength λi+1 of the incident light whenthe initial modulation frequency setting section 48 described later setsthe frequency of the modulation power source 16 at fmin. The modifiedmodulation frequency computing section 44 computes the modifiedmodulation frequency fi. The modified modulation frequency settingsection 46 sets the modified modulation frequency fi as the frequency ofthe electrical signals for modulation generated by the modulation powersource 16. The initial modulation frequency setting section 48 sets theinitial modulation frequency fmin as the frequency of the electricalsignals for modulation generated by the modulation power source 16. Theinitial modulation frequency fmin is normally set at a small value sothat the difference between the first initial phase φmin_i and thesecond initial phase φmin_i+1 may be easily contained within a range of−π to π or 0 to 2π.

[0045] Here, the principle of how the modified modulation frequencycomputing section 44 computes the modified modulation frequency fi willbe explained with reference to FIG. 2. FIG. 2(a) shows the relationshipbetween the phase and the wavelength when the modulation frequency isthe initial modulation frequency fmin. As FIG. 2(a) shows, thedifference between the first initial phase φmin_i and the second initialphase φmin_i+1 is Δφmin_i and is small. Here, when the modulationfrequency f is replaced by the modified modulation frequency fi(fi>fmin), as shown in FIG. 2(b), the difference between the firstmodified phase φi and the second modified phase φi+1 is Δφi and islarge. This is because, as shown in FIG. 2(c), Δφi and Δφmin_i areproportionate to fi/fmin. However, there will be measurement errorsunless Δφi is within the given range. In other words, in case where themeasurable range of the phase comparator 24 is between −π and π, if Δφiexceeds π, there will errors in the measurement of the phase comparator24. Therefore, when Δφi must not exceed π, in the formula of FIG. 2(c)the computation of the modified modulation frequency fi by supposingΔφi=π will give the modified modulation frequency fi as shown in FIG.2(d). If such modified modulation frequency fi is used to modulate theincident light, the difference between the first modified phase φi andthe second modified phase φi+1 will be approximately π and will begreater than the phase difference of the initial phase Δφmin_i.

[0046] And now the operation of the first embodiment will be described.FIG. 3 is a flow chart showing the operation of the first embodiment. Tobegin with, the initial modulation frequency setting section 48 sets theinitial modulation frequency fmin as the frequency of the electricalsignals for modulation generated by the modulation power source 16(S10).

[0047] And the wavelength setting section 18 sets the frequency of thevariable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated by the frequency fmin of the electricalsignals for modulation at the light modulator 14 to be supplied to theDUT 30. The transmitted light that has transmitted the DUT 30 isconverted by the optical/electrical conversion process by theoptical/electrical converter 22 to be supplied to the phase comparator24. The phase comparator 24 measures the phase differences between thephase of the electrical signals outputted by the optical/electricalconverter 22 and the phase of the electrical signals for modulationgenerated by the modulation power source 16. These phase differences arethe first initial phase φmin_i and the second initial phase φmin_i+1.

[0048] In other words, the phase comparator 24 measures the firstinitial phase φmin_i and the second initial phase φmin_i+1 (S12). Thefirst initial phase φmin_i and the second initial phase φmin_i+1 arerecorded at the initial phase recording section 42. The modifiedmodulation frequency computing section 44 reads the first initial phaseφmin_i and the second initial phase φmin_i+1 from the initial phaserecording section 42 and computes a modified modulation frequency fi(S14). The modified modulation frequency fi can be computed by using theformula shown in FIG. 2(d) when it is desired to limit Δφi at a valueequal to or less than π. If it is desired to keep Δφi at a given valueother than π, it is possible to compute the value of the modifiedmodulation frequency fi by multiplying the given value by fmin/Δφmin asshown in FIG. 2(e).

[0049] The modified modulation frequency fi is sent from the modifiedmodulation frequency computing section 44 to the modified modulationfrequency setting section 46. The modified modulation frequency settingsection 46 sets the modified modulation frequency fi as the frequency ofthe electrical signals for modulation generated by the modulation powersource 16 (S16).

[0050] Then, the wavelength setting section 18 sets the wavelength ofthe variable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated with the frequency fi of the electricalsignals for modulation at the light modulator 14 to be supplied to theDUT 30. The transmitted light having transmitted the DUT 30 is convertedby the optical/electrical conversion process by the optical/electricalconverter 22 to be supplied to the phase comparator 24. The phasecomparator 24 measures the phase differences between the phase of theelectrical signals outputted by the optical/electrical converter 22 andthe phase of the electrical signals for modulation generated by themodulation power source 16. These phase differences are the firstinitial phase φi and the second initial phase φi+1. In other words, thephase comparator 24 seeks the first modified phase φi and the secondmodified phase φi+1 (S17). The first modified phase φi and the secondmodified phase φi+1 are recorded at the modified phase recording section28. And the characteristic computing section 26 reads the first modifiedphase φi and the second modified phase φi+1 from the modified phaserecording section 28 to compute the group delay or the wavelengthdispersion of the DUT 30 (S18).

[0051] According to the first embodiment, it is possible to compute bymeans of the modified modulation frequency computing section 46 amodified modulation frequency fi that will leave the phase differencebetween the first modified phase φi and the second modified phase φi+1at a given phase value, for example at a value equal to or less than π.Therefore, if the frequency for modulating the incident light is set atthe modified modulation frequency fi by the modified modulationfrequency setting section 46, the phase difference between the firstmodified phase φi and the second modified phase φi+1 will be therequired phase value π, and thus the phase difference can be measured.Moreover, as the frequency for modulating the incident light can besufficiently large, the measure precision of the phase difference can beenhanced.

[0052] The Second Preferred Embodiment

[0053] The second preferred embodiment is different from the firstpreferred embodiment in that the modified modulation frequency fi itselfis not chosen as the frequency of the electrical signals for modulation.

[0054]FIG. 4 is a block diagram showing the configuration of an opticalcharacteristic measuring apparatus relating to the second preferredembodiment of the present invention. The optical characteristicmeasuring apparatus relating to the preferred embodiment includes alight source system 10 introducing light into the DUT 30, acharacteristic measuring system 20 for receiving the light havingtransmitted the DUT 30 and measuring the characteristics of the DUT 30,and a modulation frequency setting system 40 for setting the modulationfrequency.

[0055] The light source system 10 includes a variable wavelength lightsource 12, a light modulator 14, a modulation power source 16 and awavelength setting section 18.

[0056] The variable wavelength light source 12 generates variablewavelength light. The wavelength of the variable wavelength light variesdiscretely such as the first wavelength λi and the second wavelengthλi+1 by the operation of the wavelength setting section 18. The lightmodulator 14 modulates the variable wavelength light by the frequency ofthe electrical signals generated by the modulation power source 16 whichwill be supplied to the DUT 30. Incidentally, the light modulator 14includes LN (lithium niobate). The modulation power source 16 generateselectrical signals for modulating the frequencies set by the modulationfrequency setting system 40. The electrical signals for modulation issupplied to the light modulator 14 and the phase comparator 24 describedlater. The wavelength setting section 18 sets discretely the frequencyof variable wavelength light, for example, at the first wavelength λiand at the second wavelength λi+1.

[0057] The DUT 30 is for example an optical fiber. The incident lightsupplied to the DUT 30 transmits the DUT 30. The incident lighttransmitting the DUT 30 is called “transmitted light.”

[0058] The characteristic measuring system 20 includes anoptical/electrical converter 22, a phase comparator 24, a characteristiccomputing section 26 and a modified phase recording section 28.

[0059] The optical/electrical converter 22 converts the transmittedlight by the optical/electrical conversion process and generateselectrical signals which will then be supplied to the phase comparator24. The phase comparator 24 measures the phase difference between theelectrical signals obtained by converting by the optical/electricalconversion process the transmitted light and the electrical signals formodulation. The modified phase recording section 28 records the firstmodified phase φi and the second modified phase φi+1 respectivelycorresponding to the first wavelength λi and the second wavelength λi+1of the incident light when the modified modulation frequency settingsection 46 sets the frequency of the modulation power source 16 at anyone of fai, fbi, . . . Incidentally, fai, fbi, . . . will be describedlater. The characteristic computing section 26 computes the group delaycharacteristic and the wavelength dispersion characteristic of the DUT30 from the first modified phase φi and the second modified phase φi+1recorded in the modified phase recording section 28. The group delaycharacteristic can be computed from the relationship between the phasemeasured by the phase comparator 24 and the modified frequency (any oneof fai, fbi, . . . ). The wavelength dispersion characteristic can becomputed by differentiating the group delay characteristic by thewavelength.

[0060] The modulation frequency setting system 40 includes an initialphase recording section 42, a modified modulation frequency computingsection 44, a modified modulation frequency setting section 46, and aninitial modulation frequency setting section 48. The initial phaserecording section 42 records the first initial phase φmin_i and thesecond initial phase φmin_i+1 respectively corresponding to the firstwavelength λi and the second wavelength λi+1 of the incident light whenthe initial modulation frequency setting section 48 described later setsthe frequency of the modulation power source 16 at fmin. The modifiedmodulation frequency computing section 44 computes the modifiedmodulation frequency fi. The modified modulation frequency settingsection 46 sets the maximum below the modified modulation frequency fiwithin fai, fbi, . . . as the frequency of the electrical signals formodulation generated by the modulation power source 16. The initialmodulation frequency setting section 48 sets the initial modulationfrequencies fmin, fai, fbi, . . . as the frequency of the electricalsignals for modulation generated by the modulation power source 16.Incidentally, the initial modulation frequency fmin is normally set at asmall value so that the difference between the first initial phaseφmin_i and the second initial phase φmin_i+1 may be contained withsufficient margin within the range between −π and π or between 0 and 2π.And fai, fbi, . . . are set at a larger value than fmin. For thisreason, the initial modulation frequency fmin is called the minimuminitial modulation frequency fmin.

[0061] Here, the method by which the modified modulation frequencysetting section 46 sets the frequency of the electrical signals formodulation at any one of fai, fbi, . . . based on the modifiedmodulation frequency fi will be described with reference to FIG. 2. FIG.2(a) shows the relations between the phase and the wavelength when themodulation frequency f is the initial modulation frequency fmin. As FIG.2(a) shows, the difference between the first initial phase φmin_i andthe second initial phase φmin_i+1 is Δφmin_i and is small. When themodulation frequency f is replaced by the modified modulation frequencyfi (fi>fmin), as shown in FIG. 2(b), the difference between the firstmodified phase φi and the second modified phase φi+1 is Δφi and islarge. This is because, as shown in FIG. 2(c), Δφi and Δφmin_i areproportionate to fi/fmin. However, there will measurement errors unlessΔφi is within a given range. In other words, in case where themeasurable range of the phase comparator 24 is between −π and π, if Δφiexceeds π, there will errors in the measurement of the phase comparator24. Therefore, for example, when Δφi must not exceed π, in the formulaof FIG. 2(c) the computation of the modified modulation frequency fi bysupposing Δφi=π will give the modified modulation frequency fi as shownin FIG. 2(d).

[0062] Then, the modified modulation frequency setting section 46 setsthe maximum below the modified modulation frequency fi among fai, fbi, .. . as the modulation frequency. The modulation of the incident light bymeans of such a modulation frequency will produce the maximum value ator below π for the phase difference between the first modified phase φiand the second modified phase φi+1, which will be larger than the phasedifference Δφmin_i for the initial phase.

[0063] Then, the operation of the second embodiment will be described.FIG. 5 is a flow chart showing the operation of the second embodiment.To begin with, the initial modulation frequency setting section 48 setsthe initial modulation frequencies fmin, fai, fbi, . . . as thefrequency of the electrical signals for modulation generated by themodulation power source 16 (S11).

[0064] And the wavelength setting section 18 sets the wavelength of thevariable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated by the frequency fmin, fai, fbi, . . . ofthe electrical signals for modulation at the light modulator 14 and issupplied to the DUT 30. The transmitted light that has transmitted theDUT 30 is converted by the optical/electrical conversion process by theoptical/electrical converter 22 to be supplied to the phase comparator24. The phase comparator 24 measures the phase differences between thephase of the electrical signals outputted by the optical/electricalconverter 22 and the phase of the electrical signals for modulationgenerated by the modulation power source 16. Among these phasedifferences, the phase differences corresponding to the incident lightmodulated by the minimum initial modulation frequency fmin are the firstinitial phase φmin_i and the second initial phase φmin_i+1. And when thefrequencies of the electrical signals for modulation are fai, fbi, . . ., the phase differences measured by the phase comparator 24 are φai,φbi, . . . .

[0065] In other words, the phase comparator 24 computes the firstinitial phase φmin_i, the second initial phase φmin_i+1 and the initialphases φai, φbi, . . . (S13). The first initial phase φmin_i and thesecond initial phase φmin_i+1 are recorded in the initial phaserecording section 42. The initial phases φai, φbi, . . . are recorded inthe modified phase recording section 28.

[0066] Here, the method of computing the first initial phase φmin_i, thesecond initial phase φmin_i+1 and the initial phases φai, φbi, . . .will be described in greater detail with reference to FIG. 6. To beginwith, the wavelengths of the variable wavelength light are set at thefirst wavelength λi and the variable frequencies are switched from fminto fai, fbi, fci, . . . . And the first initial phase φmin_i and theinitial phases φai, φbi, φci, . . . are measured (S13 a). Incidentally,S13 a means the first step of S13 shown in FIG. 5. Then, the wavelengthof the variable wavelength light is set at the second wavelength ofλi+1, and the second initial phase φmin_i+1 is measured (Sl3 b). S13 bmeans the last step of S13 shown in FIG. 5.

[0067] Back in FIG. 5, the modified modulation frequency computingsection 44 reads the first initial phase φmin_i and the second initialphase φmin_i+1 from the initial phase recording section 42, and computesthe modified modulation frequency fi (S14). When it is desired to limitthe value of Δφi at a value equal to or below π, the modified modulationfrequency fi can be computed by means of the formula shown in FIG. 2(d).If it is desired to contain Δφi at a value other than π, it is possibleto compute the modified modulation frequency fi by multiplying the givenvalue by fmin/Δφmin as shown in FIG. 2(e).

[0068] The modified modulation frequency fi is sent from the modifiedmodulation frequency computing section 44 to the modified modulationfrequency setting section 46. And equally the initial modulationfrequencies fmin, fai, fbi, . . . are sent from the initial modulationfrequency setting section 48 to the modified modulation frequencysetting section 46. There, the modified modulation frequency settingsection 46 sets the maximum frequency at or below the modifiedmodulation frequency fi among the initial modulation frequencies fmin,fai, fbi, . . . as the frequency of the electrical signals formodulation generated by the modulation power source 16 (S20). Forexample, fai<fbi<fi<fci. In such a case, as shown in FIG. 6, themodified modulation frequency setting section 46 sets fbi as thefrequency of the electrical signals for modulation generated by themodulation power source 16.

[0069] Then, the wavelength setting section 18 sets the wavelength ofthe variable wavelength light generated by the variable wavelength lightsource 12 at the second wavelength λi+1. The light modulator 14 issupplied with the electrical signals for modulation generated by themodulation power source 16. The variable wavelength light is modulatedat the light modulator 14 by the frequency set by the modifiedmodulation frequency setting section 46 (any one among fai, fbi, . . . )of the electrical signals for modulation and is supplied to the DUT 30.The transmitted light that has transmitted the DUT 30 is converted bythe optical/electrical conversion process by the optical/electricalconverter 22 and is supplied to the phase comparator 24. The phasecomparator 24 measures the phase differences between the phase of theelectrical signals outputted by the optical/electrical converter 22 andthe phase of the electrical signals for modulation generated by themodulation power source 16. This phase difference is the second modifiedphase φi+1. Namely, the phase comparator 24 computes the second modifiedphase φi+1 (S22).

[0070] The method whereby the phase comparator 24 computes the secondmodified phase φi+1 will be described in greater details with referenceto FIG. 6. To begin with, let us suppose that the modified modulationfrequency setting section 46 has set fbi as the frequency of theelectrical signals for modulation generated by the modulation powersource 16. And the phase comparator 24 computes the second modifiedphase φi+1 (S22). In this case, since the second wavelength λi+1 hasbeen chosen as the wavelength of the variable wavelength light, thephase at the time when the modulation frequency is fbi and thewavelength of the variable wavelength light is λi+1 is measured. In thiscase, for any frequencies other than f=fbi, the phase at the time whenthe wavelength of the variable wavelength light is the second wavelengthλi+1 is not measured.

[0071] The second modified phase φi+1 is recorded in the modified phaserecording section 28. Here, the first modified phase φi corresponds tothe modulation frequency set by the modified modulation frequencysetting section 46 among the initial phases φai, φbi, . . . For example,if the modified modulation frequency setting section 46 has set fbi asthe modulation frequency as shown in FIG. 6, the first modified phase φiis the initial phase φbi. Therefore, we can assume that the firstmodified phase φi has already been recorded in the modified phaserecording section 28. Therefore, the characteristic computing section 26reads the first modified phase φi and the second modified phase φi+1from the modified phase recording section 28 and computes the groupdelay or the wavelength dispersion of the DUT 30 (S24).

[0072] According to the second embodiment, it is possible to compute bymeans of the modified modulation frequency computing section 46 amodified modulation frequency fi so that the phase difference betweenthe first modified phase φi and the second modified phase φi+1 could beequal to or below the given phase value, for example π. And if themaximum frequency equal to or below the modified modulating frequency fiamong the initial modulation frequencies fai, fbi, . . . is adopted forthe frequency for modulating the incident light by the modifiedmodulation frequency setting section 46, the phase difference betweenthe first modified phase φi and the second modified phase φi+1 is equalto or below the given phase value π, and therefore it is possible tomeasure the phase difference. Moreover, the possibility of choosing asufficiently wide frequency for modulating the incident light enables toenhance the precision of measurements.

[0073] In addition, the fact that the wavelength of the variablewavelength light is changed for once can economize the time required forchanging wavelength. For example, let us suppose that it takes about 3secs to change the wavelength of the variable wavelength light and about10 ms to change the modulation frequency. Then, as the time required tochange the wavelength is considerably longer than that to change themodulation frequency, it is effective to economize the time required tochange the wavelength.

[0074] The Third Preferred Embodiment

[0075] In comparison with the first and the second preferred embodimentwherein there are two wavelengths of the variable wavelength light setby the wavelength setting section 18, the third preferred embodiment isdifferent in that there are three or more wavelengths of the variablewavelength light set by the wavelength setting section 18.

[0076]FIG. 7 shows the relationship between the phase measured by thephase comparator 24 and the wavelength of the variable wavelength lightwhen there are three or more wavelengths of the variable wavelengthlight. The wavelength of the variable wavelength light varies within arange between λ1 and λn. Preferably λ1, λ2, λ3, . . . λi, λi+1, . . . λnare at equal intervals. In other words, λ2−λ1=Δλ, the first wavelengthbeing represented by λ1 and the second wavelength being represented byλ2. Then, λ2 is taken as the first wavelength and the second wavelengthλ3 is taken so that λ3−λ2=Δλ. Thereafter, this process is repeated untilλn is reached.

[0077] Such a method of setting the wavelength of variable wavelengthlight is common with the third through sixth embodiments.

[0078] The configuration of the third embodiment is similar to that ofthe first embodiment, and its description is omitted.

[0079] And now the operation of the third embodiment will be described.FIG. 8 is a flowchart showing the operation of the third embodiment. Tobegin with, the argument i for deciding the type of the wavelength λ isset at 1 (S30). Then, the question of whether the argument i plus 1 hasexceeded n is determined (S32). If the argument i plus 1 has notexceeded n (S32, No), the initial modulation frequency setting section48 sets the initial modulation frequency fmin as the frequency of theelectrical signals for modulation generated by the modulation powersource 16 (S10). And the wavelength setting section 18 sets thewavelength of the variable wavelength light generated by the variablewavelength light source 12 at the first wavelength λi and the secondwavelength λi+1. The light modulator 14 is supplied with the electricalsignals for modulation generated by the modulation power source 16. Thevariable wavelength light is modulated by the frequency fmin of theelectrical signals for modulation at the light modulator 14 and issupplied to the DUT 30. The transmitted light that has transmitted theDUT 30 is converted by the optical/electrical conversion process by theoptical/electrical converter 22 and is supplied to the phase comparator24. The phase comparator 24 measures the phase differences between thephase of the electrical signals outputted by the optical/electricalconverter 22 and the phase of the electrical signals for modulationgenerated by the modulation power source 16. These phase differences arethe first initial phase φmin_i and the second initial phase φmin_i+1.

[0080] In other words, the phase comparator 24 computes the firstinitial phase φmin_i and the second initial phase φmin_i+1 (S12). Thefirst initial phase φmin_i and the second initial phase φmin_i+1 arerecorded in the initial phase recording section 42.

[0081] And 1 is added to the argument i (S34), and the process returnsto the determination whether the argument i has exceeded n (S32). If theargument i plus 1 has exceeded n (S32, Yes), the wavelength of thevariable wavelength light has been changed, and the argument i fordeciding the type of wavelength λ will be again set at 1 (S36). Then,the process returns to the determination of whether the argument i plus1 has exceeded n (S38). If the argument i plus 1 has not exceeded n(S38, No), the modified modulation frequency computing section 44 readsthe first initial phase φmin_i and the second initial phase λmin_i+1from the initial phase recording section 42 and computes the modifiedmodulation frequency fi (S14). When it is desired to limit the value ofΔφi at a value equal to or below π, the modified modulation frequency fican be computed by using the formula shown in FIG. 2(d). When it isdesired to contain the Δφi at a given value other than π, the modifiedmodulation frequency fi can be computed by multiplying the given valueby fmin/Δφmin as shown in FIG. 2(e).

[0082] The modified modulation frequency fi is sent from the modifiedmodulation frequency computing section 44 to the modified modulationfrequency setting section 46. The modified modulation frequency settingsection 46 sets the modified modulation frequency fi as the frequency ofthe electrical signals for modulation generated by the modulation powersource 16 (S16).

[0083] Then, the wavelength setting section 18 sets the wavelength ofthe variable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated by the frequency fi of the electricalsignals for modulation at the light modulator 14 and is supplied to theDUT 30. The transmitted light that has transmitted the DUT 30 isconverted by the optical/electrical conversion process by theoptical/electrical converter 22 and is supplied to the phase comparator24. The phase comparator 24 measures the phase differences between thephase of the electrical signals outputted by the optical/electricalconverter 22 and the phase of the electrical signals for modulationgenerated by the modulation power source 16. These phase differences arethe first modified phase φi and the second modified phase φi+1. In otherwords, the phase comparator 24 computes the first modified phase φi andthe second modified phase φi+1 (S17). The first modified phase φi andthe second modified phase φi+1 are recorded in the modified phaserecording section 28.

[0084] Then, 1 is added to the argument i (S40) and the process returnsto the determination of whether the argument i has exceeded n (S38). Ifthe argument i plus 1 has exceeded n (S38, Yes), the wavelength of thevariable wavelength light has already changed. Therefore, thecharacteristic computing section 26 reads the first modified phase φiand the second modified phase φi+1 from the modified phase recordingsection 28, and computes the group delay or the wavelength dispersion ofthe DUT 30 (S18).

[0085] According to the third preferred embodiment, it is possible tomeasure the characteristics of the DUT 30 even if three or morewavelengths of the variable wavelength light are changed.

[0086] The Fourth Preferred Embodiment

[0087] In comparison with the first and the second embodiments whereinthe wavelength setting section 18 sets two wavelengths of the variablewavelength light, the fourth embodiment is different in that it hasthree or more wavelengths of the variable wavelength light set by thewavelength setting section 18.

[0088] Also in comparison with the third preferred embodiment whereinthe modified modulation frequency is computed after all the firstwavelength λi and the second wavelength λi+1 have been set, in otherwords after all the initial phases have been measured, the fourthpreferred embodiment is different in that the modified modulationfrequency is computed after each setting of the first wavelength λi andthe second wavelength λi+1.

[0089] The configuration of the fourth preferred embodiment is similarto that of the first embodiment and its description is omitted.

[0090] And now the operation of the fourth preferred embodiment will bedescribed. FIG. 9 is a flowchart showing the operation of the fourthpreferred embodiment. To begin with, the argument i for deciding thetype of the wavelength λ is set at 1 (S30). Then, the question ofwhether the argument i plus 1 has exceeded n is determined (S32). If theargument i plus 1 has not exceeded n (S32, No), the initial modulationfrequency setting section 48 sets the initial modulation frequency fminas the frequency of the electrical signals for modulation generated bythe modulation power source 16 (S10). And the wavelength setting section18 sets the wavelength of the variable wavelength light generated by thevariable wavelength light source 12 at the first wavelength λi and thesecond wavelength λi+1. The light modulator 14 is supplied with theelectrical signals for modulation generated by the modulation powersource 16. The variable wavelength light is modulated by the frequencyfmin of the electrical signals for modulation at the light modulator 14and is supplied to the DUT 30. The transmitted light that hastransmitted the DUT 30 is converted by the optical/electrical conversionprocess by the optical/electrical converter 22 and is supplied to thephase comparator 24. The phase comparator 24 measures the phasedifferences between the phase of the electrical signals outputted by theoptical/electrical converter 22 and the phase of the electrical signalsfor modulation generated by the modulation power source 16. These phasedifferences are the first initial phase φmin_i and the second initialphase φmin_i+1.

[0091] In other words, the phase comparator 24 computes the firstinitial phase φmin_i and the second initial phase φmin_i+1 (S12). Thefirst initial phase φmin_i and the second initial phase φmin_i+1 arerecorded in the initial phase recording section 42.

[0092] The modified modulation frequency computing section 44 reads thefirst initial phase φmin_i and the second initial phase φmin_i+1 fromthe initial phase recording section 42 and computes the modifiedmodulation frequency fi (S14). When it is desired to limit the value ofΔφi at a value equal to or below π, the modified modulation frequency fican be computed by means of the formula shown in FIG. 2(d). If it isdesired to keep Δφi at a value other than π, it is possible to computethe modified modulation frequency fi by multiplying the given value byfmin/Δφmin as shown in FIG. 2(e).

[0093] The modified modulation frequency fi is sent from the modifiedmodulation frequency computing section 44 to the modified modulationfrequency setting section 46. The modified modulation frequency settingsection 46 sets the modified modulation frequency fi as the frequency ofthe electrical signals for modulation generated by the modulation powersource 16 (S16).

[0094] Then, the wavelength setting section 18 sets the wavelength ofthe variable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated by the frequency fi of the electricalsignals for modulation at the light modulator 14 and is supplied to theDUT 30. The transmitted light that has transmitted the DUT 30 isconverted by the optical/electrical conversion process by theoptical/electrical converter 22 and is supplied to the phase comparator24. The phase comparator 24 measures the phase differences between thephase of the electrical signals outputted by the optical/electricalconverter 22 and the phase of the electrical signals for modulationgenerated by the modulation power source 16. Theses phase differencesare the first modified phase φi and the second modified phase φi+1. Inother words, the phase comparator 24 computes the first modified phaseφi and the second modified phase φi+1 (S17). The first modified phase φiand the second modified phase φi+1 are recorded in the modified phaserecording section 28.

[0095] Then, 1 is added to the argument i (S34) and the process returnsto the determination of whether the argument i has exceeded n (S30). Ifthe argument i plus 1 has exceeded n (S32, Yes), the wavelength of thevariable wavelength light has already changed. Therefore, thecharacteristic computing section 26 reads the first modified phase φiand the second modified phase φi+1 from the modified phase recordingsection 28, and computes the group delay or the wavelength dispersion ofthe DUT 30 (S18).

[0096] According to the fourth preferred embodiment, it is possible tomeasure the characteristics of the DUT 30 even if three or morewavelengths of the variable wavelength light are changed.

[0097] The Fifth Preferred Embodiment

[0098] In comparison with the first and the second preferred embodimentswherein the wavelength setting section 18 sets two wavelengths of thevariable wavelength light, the fifth preferred embodiment is differentin that it has three or more wavelengths of the variable wavelengthlight set by the wavelength setting section 18.

[0099] The configuration of the fifth preferred embodiment is similar tothat of the second embodiment and its description is omitted.

[0100] And now the operation of the fifth preferred embodiment will bedescribed. FIG. 10 is a flowchart showing the operation of the fifthpreferred embodiment. To begin with, the argument i for deciding thetype of the wavelength λ is set at 1 (S30). Then, the question ofwhether the argument i plus 1 has exceeded n is determined (S32). If theargument i plus 1 has not exceeded n (S32, No), the initial modulationfrequency setting section 48 sets the initial modulation frequenciesfmin, fai, fbi, . . . as the frequencies of the electrical signals formodulation generated by the modulation power source 16 (S11).

[0101] And the wavelength setting section 18 sets the wavelength of thevariable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated by the frequencies fmin, fai, fbi, . . .of the electrical signals for modulation at the light modulator 14 andis supplied to the DUT 30. The transmitted light that has transmittedthe DUT 30 is converted by the optical/electrical conversion process bythe optical/electrical converter 22 and is supplied to the phasecomparator 24. The phase comparator 24 measures the phase differencesbetween the phase of the electrical signals outputted by theoptical/electrical converter 22 and the phase of the electrical signalsfor modulation generated by the modulation power source 16. Among thesephase differences, the phase differences corresponding to the incidentlight modulated by the minimum initial modulation frequency fmin are thefirst initial phase φmin_i and the second initial phase φmin_i+1.Further, the phase differences measured by the phase comparator 24 whenthe wavelength of the variable wavelength light is the first wavelengthλi and the frequencies of the electrical signals for modulation are fai,fbi, . . . are φai, φbi,

[0102] In other words, the phase comparator 24 computes the firstinitial phase φmin_i, the second initial phase φmin_i+1 and the initialphases φai, φbi, . . . (S13). The first initial phase φmin_i and thesecond initial phase φmin_i+1 are recorded in the initial phaserecording section 42. The initial phases φai, φbi, . . . are recorded inthe modified phase recording section 28.

[0103] Here, the method of computing the first initial phase φmin_i, thesecond initial phase φmin i+1 and the initial phases φai, φbi, . . .will be described in greater detail with reference to FIG. 6. To beginwith, the frequencies of the variable wavelength light are set at thefirst wavelength λi and the variable frequencies are switched from fminto fai, fbi, fci, . . . . And the first initial phase φmin_i and theinitial phases φai, φbi, φci, . . . are measured (S13 a). Here, S13 ameans the first step of S13 shown in FIG. 5. Then, the frequency of thevariable wavelength light is set at the second wavelength λi+1, and thesecond initial phase φmin i+1 is measured (S13 b). Here, S13 b means thelast step of S13 shown in FIG. 5.

[0104] Then, 1 is added to the argument i (S34) and the process returnsto the determination of whether the argument i has exceeded n (S32). Ifthe argument i plus 1 has exceeded n (S32, Yes), the wavelength of thevariable wavelength light has already changed, and the argument i fordeciding the type of wavelength λ is again set at 1 (S36). Then, theprocess returns to the determination of whether the argument i plus 1has exceeded n (S38). If the argument i plus 1 has not exceeded n (S38,No), the modified modulation frequency computing section 44 reads thefirst initial phase φmin_i and the second modified phase φmin_i+1 fromthe initial phase recording section 42 and computes the modifiedmodulation frequency fi (S 14). When it is desired to limit the value ofΔφi at a value equal to or below π, the modified modulation frequency fican be computed by using the formula shown in FIG. 2(d). When it isdesired to contain the Δφi at a given value other than π, the modifiedmodulation frequency fi can be computed by multiplying the given valueby fmin/Δφmin as shown in FIG. 2(e).

[0105] The modified modulation frequency fi is sent from the modifiedmodulation frequency computing section 44 to the modified modulationfrequency setting section 46. And equally the initial modulationfrequencies fmin, fai, fbi, . . . are sent from the initial modulationfrequency setting section 48 to the modified modulation frequencysetting section 46. There, the modified modulation frequency settingsection 46 sets the maximum frequency at or below the modifiedmodulation frequency fi among the initial modulation frequencies fmin,fai, fbi, . . . as the frequency of the electrical signals formodulation generated by the modulation power source 16 (S20). Forexample, fai<fbi<fi<fci. In such a case, as shown in FIG. 6, themodified modulation frequency setting section 46 sets fbi as thefrequency of the electrical signals for modulation generated by themodulation power source 16.

[0106] Then, the wavelength setting section 18 sets the wavelength ofthe variable wavelength light generated by the variable wavelength lightsource 12 at the second wavelength λi+1. The light modulator 14 issupplied with the electrical signals for modulation generated by themodulation power source 16. The variable wavelength light is modulatedby the frequency (any one of fai, fbi, . . . ) set by the modifiedmodulation frequency setting section 46 at the light modulator 14 and issupplied to the DUT 30. The transmitted light that has transmitted theDUT 30 is converted by the optical/electrical conversion process by theoptical/electrical converter 22 and is supplied to the phase comparator24. The phase comparator 24 measures the phase differences between thephase of the electrical signals outputted by the optical/electricalconverter 22 and the phase of the electrical signals for modulationgenerated by the modulation power source 16. This phase difference isthe second modified phase φi+1. In other words, the phase comparator 24computes the second modified phase φi+1 (S22). At this point, the secondmodified phase φi+1 is recorded at the modified phase recording section28.

[0107] The method whereby the phase comparator 24 computes the secondmodified phase φi+1 will be described in greater details with referenceto FIG. 6. To begin with; let us suppose that the modified modulationfrequency setting section 46 has set fbi as the frequency of theelectrical signals for modulation generated by the modulation powersource 16. And the phase comparator 24 computes the second modifiedphase φi+1 (S22). In this case, since the second wavelength λi+1 hasbeen chosen as the wavelength of the variable wavelength light, thephase at the time when the modulation frequency is fbi and thewavelength of the variable wavelength light is λi+1 will be measured. Inthis case, for any frequencies other than f=fbi, the phase at the timewhen the wavelength of the variable wavelength light is the secondwavelength λi+1 will not be measured.

[0108] Then, 1 is added to the argument i (S40) and the process returnsto the determination of whether the argument i has exceeded n (S38). Ifthe argument i plus 1 has exceeded n (S38, Yes), the wavelength of thevariable wavelength light has already changed. Now, the first modifiedphase φi corresponds to the modulation frequency set by the modifiedmodulation frequency setting section 46 among the initial phase φai,φbi, . . . set by the modified modulation frequency setting section 46.For example, if the modified modulation frequency setting section 46sets fbi as the modulation frequency as shown in FIG. 6, the firstmodified phase φi will be the initial phase φbi. Thus, it can be saidthat the first modified phase φi has already been recorded in themodified phase recording section 28. Then, the characteristic computingsection 26 reads the first modified phase φi and the second modifiedphase φi+1 from the modified phase recording section 28, and computesthe group delay or the wavelength dispersion of the DUT 30 (S24).

[0109] According to the fifth preferred embodiment, it is possible tomeasure the characteristics of the DUT 30 even if three or morewavelengths of the variable wavelength light are changed.

[0110] The Sixth Preferred Embodiment

[0111] In comparison with the first and the second preferred embodimentswherein the wavelength setting section 18 sets two wavelengths of thevariable wavelength light, the sixth preferred embodiment is differentin that it has three or more wavelengths of the variable wavelengthlight set by the wavelength setting section 18.

[0112] Also in comparison with the fifth preferred embodiment whereinthe modified modulation frequency is computed after all the firstwavelength λi and the second wavelength λi+1 have been set, in otherwords after all the initial phases have been measured, the sixthpreferred embodiment is different in that the modified modulationfrequency is computed after each setting of the first wavelength λi andthe second wavelength λi+1.

[0113] The configuration of the sixth preferred embodiment is similar tothat of the second embodiment and its description is omitted.

[0114] And now the operation of the sixth preferred embodiment will bedescribed. FIG. 11 is a flowchart showing the operation of the sixthpreferred embodiment. To begin with, the argument i for deciding thetype of the wavelength λ is set at 1 (S30). Then, the question ofwhether the argument i plus 1 has exceeded n is determined (S32). If theargument i plus 1 has not exceeded n (S32, No), the initial modulationfrequency setting section 48 sets the initial modulation frequenciesfmin, fai, fbi, . . . as the frequencies of the electrical signals formodulation generated by the modulation power source 16 (S11).

[0115] And the wavelength setting section 18 sets the wavelength of thevariable wavelength light generated by the variable wavelength lightsource 12 at the first wavelength λi and the second wavelength λi+1. Thelight modulator 14 is supplied with the electrical signals formodulation generated by the modulation power source 16. The variablewavelength light is modulated by the frequencies fmin, fai, fbi, . . .of the electrical signals for modulation at the light modulator 14 andis supplied to the DUT 30. The transmitted light that has transmittedthe DUT 30 is converted by the optical/electrical conversion process bythe optical/electrical converter 22 to be supplied to the phasecomparator 24. The phase comparator 24 measures the phase differencesbetween the phase of the electrical signals outputted by theoptical/electrical converter 22 and the phase of the electrical signalsfor modulation generated by the modulation power source 16. Among thesephase differences, the phase differences corresponding to the incidentlight modulated by the minimum initial modulation frequency fmin are thefirst initial phase φmin_i and the second initial phase φmin_i+1.Further, the phase differences measured by the phase comparator 24 whenthe wavelength of the variable wavelength light is the first wavelengthλi and the frequencies of the electrical signals for modulation are fai,fbi, . . . are φai, φbi, . . .

[0116] In other words, the phase comparator 24 computes the firstinitial phase φmin_i, the second initial phase φmin_i+1 and the initialphases φai, φbi, . . . (S13). The first initial phase φmin_i and thesecond initial phase φmin_i+1 are recorded in the initial phaserecording section 42. The initial phases φai, φbi, . . . are recorded inthe modified phase recording section 28.

[0117] Here, the method of computing the first initial phase φmin_i, thesecond initial phase φmin_i+1 and the initial phases φai, φbi, . . .will be described in greater detail with reference to FIG. 6. To beginwith, the frequencies of the variable wavelength light are set at thefirst wavelength λi and the modulation frequencies are switched fromfmin to fai, fbi, fci, . . . And the first initial phase φmin_i and theinitial phases φai, φbi, φci, . . . are measured (S13 a). Here, S13 ameans the first step of S13 shown in FIG. 5. Then, the wavelength of thevariable wavelength light is set at the second wavelength of λi+1, andthe second initial phase φmin_i+1 is measured (S13 b). Here, S13 b meansthe last step of S13 shown in FIG. 5.

[0118] The modified modulation frequency computing section 44 reads thefirst initial phase φmin_i and the second initial phase φmin_i+1 fromthe initial phase recording section 42 and computes the modifiedmodulation frequency fi (S14). When it is desired to limit the value ofΔφi at a value equal to or below π, the modified modulation frequency fican be computed by means of the formula shown in FIG. 2(d). If it isdesired to keep Δφi at a value other than π, it is possible to computethe modified modulation frequency fi by multiplying the given value byfmin/Δφmin as shown in FIG. 2(e).

[0119] The modified modulation frequency fi is sent from the modifiedmodulation frequency computing section 44 to the modified modulationfrequency setting section 46. And equally the initial modulationfrequencies fmin, fai, fbi, . . . are sent from the initial modulationfrequency setting section 48 to the modified modulation frequencysetting section 46. Therefore, the modified modulation frequency settingsection 46 sets the maximum frequency at or below the modifiedmodulation frequency fi among the initial modulation frequencies fmin,fai, fbi, . . . as the frequency of the electrical signals formodulation generated by the modulation power source 16 (S20). Forexample, fai<fbi<fi<fci. In such a case, as shown in FIG. 6, themodified modulation frequency setting section 46 sets fbi as thefrequency of the electrical signals for modulation generated by themodulation power source 16.

[0120] Then, the wavelength setting section 18 sets the wavelength ofthe variable wavelength light generated by the variable wavelength lightsource 12 at the second wavelength λi+1. The light modulator 14 issupplied with the electrical signals for modulation generated by themodulation power source 16. The variable wavelength light is modulatedby the frequency (any one of fai, fbi, . . . ) of the electrical signalsfor modulation at the light modulator 14 and is supplied to the DUT 30.The transmitted light that has transmitted the DUT 30 is converted bythe optical/electrical conversion process by the optical/electricalconverter 22 to be supplied to the phase comparator 24. The phasecomparator 24 measures the phase differences between the phase of theelectrical signals outputted by the optical/electrical converter 22 andthe phase of the electrical signals for modulation generated by themodulation power source 16. This phase difference is the second modifiedphase φi+1. In other words, the phase comparator 24 computes the secondmodified phase φi+1 (S22). At this point, the second modified phase φi+1is recorded at the modified phase recording section 28.

[0121] The method whereby the phase comparator 24 computes the secondmodified phase φi+1 will be described in greater details with referenceto FIG. 6. To begin with, let us suppose that the modified modulationfrequency setting section 46 has set fbi as the frequency of theelectrical signals for modulation generated by the modulation powersource 16. And the phase comparator 24 computes the second modifiedphase φi+1 (S22). In this case, since the second wavelength λi+1 hasbeen chosen as the wavelength of the variable wavelength light, thephase at the time when the modulation frequency is fbi and thewavelength of the variable wavelength light is λi+1 will be measured. Inthis case, for any frequencies other than f=fbi, the phase at the timewhen the wavelength of the variable wavelength light is the secondwavelength λi+1 will not be measured.

[0122] Then, 1 is added to the argument i (S34) and the process returnsto the determination of whether the argument i has exceeded n (S32). Ifthe argument i plus 1 has exceeded n (S32, Yes), the wavelength of thevariable wavelength light has already changed. Now, the first modifiedphase φi corresponds to the modulation frequency among the initial phaseφai, φbi, . . . set by the modified modulation frequency setting section46. For example, if the modified modulation frequency setting section 46sets fbi as the modulation frequency as shown in FIG. 6, the firstmodified phase φi will be the initial phase φbi. Thus, it can be saidthat the first modified phase φi has already been recorded in themodified phase recording section 28. Then, the characteristic computingsection 26 reads the first modified phase φi and the second modifiedphase φi+1, and computes the group delay or the wavelength dispersion ofthe DUT 30 (S24).

[0123] According to the sixth preferred embodiment, it is possible tomeasure the characteristics of the DUT 30 even if three or morewavelengths of the variable wavelength light are changed.

[0124] In the meanwhile, the embodiment described above can be realizedby having a media reading apparatus of a computer provided with a CPU, ahard disk, memory media (a floppy disk, a CD-ROM, etc.) read a programexecuting various functions described above and installing the programon a hard disk. In this way, the functions described above can beperformed.

[0125] According to the present invention, the range of frequencies formodulating the incident light can be enlarged and therefore theprecision of measuring can be enhanced.

7. (amended) the optical characteristic measuring apparatus according toclaim 1, comprising n optical/electrical conversion means for outputtingelectrical signals obtained by optical/electrical conversion of saidtransmitted light to said phase measuring means.
 8. (Amended) Theoptical characteristic measuring apparatus according to claim 1, whereinsaid phase measuring means measures the phase difference between saidmodulating signal and said transmitted light.
 9. (Amended) The opticalcharacteristic measuring apparatus according to claim 1, comprising acharacteristic computing means for computing the group delay or thewavelength dispersion of said device under test by means of said phasedifference measured by said phase measuring means.
 1. An apparatus formeasuring the characteristics of device under test that transmits lightcomprising: a variable wavelength light source for generating a variablewavelength light; a wavelength setting means for setting said variablewavelength light at a first wavelength and a second wavelength; aninitial modulation frequency setting means for setting the initialmodulation frequency for modulation; a modulating signal generatingmeans for generating a modulating signal of a set modulation frequency;an optical modulating means for receiving the input of said modulatingsignal and modulating said variable wavelength light with the frequencyof said modulating signal; a phase measuring means for measuring a firstphase of a transmitted light, which is obtained by the transmissionthrough the device under test of an incident light having the firstwavelength and a second phase of said transmitted light, which isobtained by the transmission through the device under test of anincident light having the second wavelength; a modified modulationfrequency computing means for computing a modified modulation frequencyby multiplying the value, which is obtained by dividing the given phasevalue by the phase difference between the first phase and the secondphase, by said initial modulation frequency; and a modified modulationfrequency setting means for setting said modified modulation frequencyas the frequency of said modulating signal, wherein the characteristicsof device under test are measured on the basis of the transmitted lightresulting from the transmission through said device under test of theincident light modulated by a frequency set by said modified modulationfrequency setting means.
 2. The optical characteristic measuringapparatus according to claim 1, wherein said initial modulationfrequency setting means sets the minimum initial modulation frequencyand said initial modulation frequencies other than said minimum initialmodulation frequency; said modified modulation frequency computing meanscomputes a modified modulation frequency by multiplying by said minimuminitial modulation frequency the value obtained by dividing said givenphase value by the phase difference between said first phase and saidsecond phase of said transmitted light resulting from the transmissionthrough said device under test of said incident light modulated by saidminimum initial modulation frequency; and said modified modulationfrequency setting means sets the maximum said initial modulationfrequency among said initial modulation frequencies equal to or belowsaid modified modulation frequencies as the frequency of said modulatingsignal.
 3. The optical characteristic measuring apparatus according toclaim 1 or 2, wherein there are a plurality of first wavelengths and aplurality of second wavelengths.
 4. The optical characteristic measuringapparatus according to claim 3 wherein, the intervals between said firstwavelength and said second wavelength are equal, and said secondwavelength is taken as said first wavelength and furthermore anothersecond wavelength is taken so that the intervals between said firstwavelength and said second wavelength are equal.
 5. The opticalcharacteristics measuring apparatus according to claim 3 or 4, whereinafter completing the setting of said first wavelength and said secondwavelength, said modified modulation frequency setting means sets saidmodified modulation frequency as the frequency of said modulatingsignal.
 6. The optical characteristics measuring apparatus according toclaim 3 or 4, wherein every time when said first wavelength and saidsecond wavelength are set, said modified modulation frequency settingmeans sets said modified modulation frequency as the frequency of saidmodulating signal.
 7. The optical characteristics measuring apparatusaccording to either one of claims 1 to 6, comprising anoptical/electrical conversion means for outputting electrical signalsobtained by optical/electrical conversion of said transmitted light tosaid phase measuring means.
 8. The optical characteristics measuringapparatus according to either one of claims 1 to 6, wherein said phasemeasuring means measures the phase difference between said modulatingsignal and said transmitted light.
 9. The optical characteristicsmeasuring apparatus according to either one of claims 1 to 6, comprisinga characteristic computing means for computing the group delay or thewavelength dispersion of said device under test by means of said phasedifference measured by said phase measuring means.
 10. A method formeasuring the characteristics of device under test that transmits lightcomprising: a variable wavelength light generating step for generating avariable wavelength light; a wavelength setting step for setting saidvariable wavelength light at a first wavelength and a second wavelength;an initial modulation frequency setting step for setting the initialmodulation frequency for modulation; a modulating signal generating stepfor generating a modulating signal of a set modulation frequency; anoptical modulating step for receiving the input of said modulatingsignal and modulating said variable wavelength light with the frequencyof said modulating signal; a phase measuring step for measuring thefirst phase of said transmitted light, which is obtained by thetransmission through the device under test of an incident light havingthe first wavelength and the second phase of said transmitted light,which is obtained by the transmission through the device under test ofan incident light having the second wavelength; a modified modulationfrequency computing step for computing a modified modulation frequencyby multiplying the value, which is obtained by dividing the given phasevalue by the phase difference between the first phase and the secondphase, by said initial modulation frequency; and a modified modulationfrequency setting step for setting said modified modulation frequency asthe frequency of said modulating signal, wherein the characteristics ofdevice under test are measured on the basis of the transmitted lightresulting from the transmission through said device under test of theincident light modulated by a frequency set by said modified modulationfrequency setting step.
 11. The optical characteristic measuring methodaccording to claim 10, wherein said initial modulation frequency settingstep sets the minimum initial modulation frequency and said initialmodulation frequencies other than said minimum initial modulationfrequency; said modified modulation frequency computing step computes amodified modulation frequency by multiplying by said minimum initialmodulation frequency the value obtained by dividing said given phasevalue by the phase difference between said first phase and said secondphase of said transmitted light resulting from the transmission throughsaid device under test of said incident light modulated by said minimuminitial modulation frequency; and said modified modulation frequencysetting step sets the maximum said initial modulation frequency amongsaid initial modulation frequencies equal to or below said modifiedmodulation frequencies as the frequency of said modulating signal.
 12. Acomputer-readable medium having a program of instructions for executionby the computer to perform a characteristics measuring process formeasuring characteristics of device under test that transmits light,said characteristics measuring process comprising: a variable wavelengthlight generating process for generating a variable wavelength light; awavelength setting process for setting said variable wavelength light ata first wavelength and a second wavelength; an initial modulationfrequency setting process for setting the initial modulation frequencyfor modulation; a modulating signal generating process for generating amodulating signal of a set modulation frequency; an optical modulatingprocess for receiving the input of said modulating signal and modulatingsaid variable wavelength light with the frequency of said modulatingsignal; a phase measuring process for measuring the first phase of saidtransmitted light, which is obtained by the transmission through thedevice under test of an incident light having the first wavelength andthe second phase of said transmitted light, which is obtained by thetransmission through the device under test of an incident light havingthe second wavelength; a modified modulation frequency computing processfor computing a modified modulation frequency by multiplying the value,which is obtained by dividing the given phase value by the phasedifference between the first phase and the second phase, by said initialmodulation frequency; and a modified modulation frequency settingprocess for setting said modified modulation frequency as the frequencyof said modulating signal, wherein the characteristics of device undertest are measured on the basis of the transmitted light resulting fromthe transmission through said device under test of the incident lightmodulated by a frequency set by said modified modulation frequencysetting process.
 13. The computer-readable medium according to claim 12,wherein said initial modulation frequency setting process sets theminimum initial modulation frequency and said initial modulationfrequencies other than said minimum initial modulation frequency; saidmodified modulation frequency computing process computes a modifiedmodulation frequency by multiplying by said minimum initial modulationfrequency the value obtained by dividing said given phase value by thephase difference between said first phase and said second phase of saidtransmitted light resulting from the transmission through said deviceunder test of said incident light modulated by said minimum initialmodulation frequency; and said modified modulation frequency settingprocess sets the maximum said initial modulation frequency among saidinitial modulation frequencies equal to or below said modifiedmodulation frequencies as the frequency of said modulating signal.